1. Field of the Invention
The invention relates to a composite membrane useful as a scavenger of unwanted gas components which cause false-positive detection of organic nerve gas agents in sensors employed for such detection as, for example, by military personnel.
2. Description of Related Art
Organic nerve gas agent sensing devices are employed by military personnel to provide early warning of deadly gases. Such devices are extremely sensitive, being capable of detecting gas concentrations as low as 0.2 micrograms per liter. In addition, in certain industrial applications, the detection of minute concentrations of gaseous components is critical.
In sensing and signaling the presence of certain gases, because of the high sensitivity of sensors used to detect such small concentrations of gas, the presence of other gases in the atmosphere can result in false-positive signals. For example, the presence of small amounts of hydrogen sulfide or hydrogen cyanide gas can set off the alarm in an electronic detecting device which is very sensitively adjusted to detect and signal the presence of minute amounts of phosphorus-containing organic nerve gas agents. Heretofore, these interfering gases have been removed for a time by a scavenging membrane installed within the sensing device.
The conventional scavenging membrane is a composite of polystyrene divinylbenzene copolymer microspheres, whose surfaces are treated to introduce sulfonic acid groups, which microspheres are imbedded in a porous, nonwoven nylon matrix and bound with a polyacrylonitrile/polyvinyl chloride coating. The exposed sulfonic acid groups are ion-exchanged to form the silver salt. The function of the silver salt is to react with and remove the unwanted, interfering gases, such as hydrogen sulfide or hydrogen cyanide, while simultaneously permitting even very minute amounts of the nerve gas agent to be detected to diffuse through the membrane to the sensor.
There are several deficiencies in this scavenging system which contribute to a relatively short active life of the sensor. There is some loss in sensor activity because the nerve gas agent will interact with the styrene divinylbenzene microspheres, with the non-perfluoro organic polymer matrix, or with absorbed water. More importantly, the period of scavenging protection provided is relatively short because a great number, perhaps most of the sulfonic acid groups, are buried in the organic polymer matrix and binder and, consequently, are not available either for silver salt exchange or for subsequent reaction with interfering gases.
The present invention overcomes most of the deficiencies of the conventional scavenging membrane and, suprisingly, has a useful life which can be twice that of the conventional membrane.